Available chlorine single-stage bleaching of sulfate pulp



AVAILABLE CHLORINE SINGLE-STAGE BLEACHING OF SULFATE PULP William A.Stone, New Orleans, La., and John George Matarese, Evanston, 111.,assignors to Olin Mathlesou Chemical Corporation, a corporation ofVirginia N Drawing. Application March 26, 1952, Serial No. 278,748

2 Claims. (C1. 8-105) Our invention relates to improvements in themultistage bleaching of sulfate cellulose pulps and more particularly toa bleaching operation employing a combination of chlorine dioxide,particularly in the form of a water soluble chlorite, and chlorine inthe first or preliminary bleaching stage.

In the conventional methods of multi-stage bleaching of sulfatecellulose pulps, chlorine has been a commonly used reagent. While thisreagent, used most frequently in conjunction with hypochlorites, willproduce a satisfactory color, it tends to impair the strength of thesulfate pulp, the tendency being particularly marked in cases where ahigh degree of bleaching is required. The strong oxidizing power ofchlorine and hypochlorite is apparently a chief cause of thisundesirable degradation, although direct reaction of the chlorine andsulfate pulp may be partially responsible.

In the bleaching of sulfate pulp many variations in procedure arepossible. However, the minimum operation ordinarily consists of at leastthree stages, the first of which is a chlorination stage where the pulpis treated with chlorine at an acid pH for from about five to tenminutes. Under these conditions the lignin is largely chlorinated due tothe presence in the acid solution of most of the chlorine as dissolvedelemental chlorine. Oxidation is avoided by the acidity of the solutionwhich restricts the formation of hypochlorous acid, the oxidizing agentin the system. Furthermore, since the chlorination reaction isrelatively rapid as compared to the oxidation reaction, limiting thetime of contact also assists in avoiding oxidation.

In a typical operation, pump consistency is about 1.5 to 3 per cent andthe preliminary chlorine bleach is carried out at about 70 F. Thechlorinated pulp is extracted in a second stage with caustic, forexample with 2 per cent caustic, at 120 to 140 F. using a 10 per centpulp consistency. The residual from the caustic extraction stage ismaintained at about 0.9 gram per liter of available chlorine and ispassed to a tile tank and held for about one hour at a temperature ofabout 60 F. The residual from the hold tank with about 0.2 gram perliter of available chlorine is then passed to a first washer, repulpedwith hot water and a hypochlorite and passed to a third stage whichcomprises the beating stage. Additional hypochlorite is added to thebeaters and the pulp beaten for about an hour at 90 F. prior to beingpassed to a second washer. Brightnesses of the bleached pulp averagebetween 68 and 70 per cent but strengths are poor.

While chlorine dioxide and chlorites have been widely used in bleachingsulfate pulps to obtain high brightness without decrease in strength,tearing or fold resistance, their use has ordinarily been restricted bythe relatively high cost of chlorites as compared with chlorine tostages subsequent to the first or early chlorine bleach stage where theprincipal advantage of the chlorine dioxide and chlorites are utilizedto the maximum degree.

nited States Patent 0 2,741,537 Patented Apr. 10, 1956 We have now foundthat when chlorine dioxide and chlorine are combined in particularproportions in the first or preliminary stage in bleaching a sulfatepulp, substantial advantages for the bleaching operation, particularlyin chlorine savings, may be realized. For example, we have found thatthe combination chlorine dioxidechlorine bleach unexpectedly reduces thetotal available chlorine requirement of the pulp to a substantialextent, e. g. by 20 to 50 per cent. In addition, substantial savings maybe realized by using less caustic for neutralization, by elimination ofbleaching in and operation of the beaters, by elimination of a secondstage washer, and by reduction in the amounts of pigments used forbrightness and opacity. Equivalent or higher brightnesses, greaterstrength retention, easier and faster bleaching and less color reversionare also obtained.

Our invention resides in an improvement in the process of bleachingsulfate cellulose pulps in which the pulp to be bleached is contacted inaqueous medium with chlorine, neutralized, and washed. In the process ofour invention the pulp to be bleached is contacted in aqueous mediumwith chlorine and chlorine dioxide, the mole ratio of chlorine tochlorine dioxide being within the range of about 2.5 to 3.5: l.Advantageously the pulp in aqueous medium is contacted with chlorine anda water soluble chlorite at a pH of about 2.5 to 2.7, the mole ratio ofchlorine to chlorite being within the range of about 3.5 to 4.5. Of thewater soluble chlorites, we prefer to use sodium chlorite.

The operation and advantages of the processes of our invention will befurther illustrated by the following examples.

Example I Tests were conducted using 20 pounds of sodium chlorite andpounds of chlorine per ton of sulfate pulp in the preliminary orchlorination stage. After neutralization the pH was maintained at around9 to 9.5 in order to keep the pulp slightly alkaline at the end of 1hour in the hold tank. Brightness of pulp off the first washer averagedaround 68 per cent. Five pounds of sodium chlorite per ton of pulp wereadded to the heaters for one hour at F. Brightness of pulp off thesecond washer averaged 74 per cent with good strength. However, thechlorine dioxide odor from the beaters and the second washer wasobjectionable to the operators.

Example II e Bleached Stock Brown Stool. (1 Stage only) Beating TimeMullen Tear Mullen Tear Percent Percent 0 minutes l7. 1 1. G0 24. 8 2.44 20 minutes 63. 9 l. 85 89. 4 2. S6 40 minutes 87. 2 3. 13 105. 6 2.45 60 minutes 09. 6 2. 75 127. 2 2. 34

Fadometer tests indicated about the same reversion (8%) for the onestage bleached pulp as for the two-stage bleached pulp on oiled paperwhen original brightness was 72% for both. Titanox added to the heaterswas reduced from 39 pounds per ton for the two-stage bleached pulp to 10pounds per ton for the one-stage bleached pulp.

We claim:

1. A single stage bleaching operation for sulfate cellulose pulps inwhich the only bleaching operation necessary to afiord products ofacceptable color consists essentially of simultaneously contacting thepulp in aqueous medium under bleaching conditions with chlorine andchlorine dioxide, the molar ratio of chlorine to chlorine dioxide beingwithin the range of about 2.5 to about 3.5:1.

2. A single stage bleaching operation for sulfate cellulose pulps inwhich the only bleaching operation necessary to afford products ofacceptable color consists essentially of simultaneously contacting thepulp in aqueous medium under bleaching conditions with chlorine and awater soluble chlorite, the molar ratio of chlorine to the water solublechlorite being within the range of about 3.5 to about 4.5:1.

References Cited in the file of this patent UNITED STATES PATENTS2,129,719 Vincent Sept. 13, 1938 2,166,330 Vincent July 18, 19392,235,837 Logan Mar. 25, 1941 FOREIGN PATENTS 191,357 Great Britain Mar.22, 1923 446,444 Belgium July 15, 1942 OTHER REFERENCES Staff Report,Chem. and Eng. News, March 17, 1947, pg. 766.

Woodside et al.: Condensation of Round Table Discussion on PulpBleaching. Paper Trade Journal, December 9, 1948, pgs. 21-25, especiallyat page 24, col. 2.

Sen Gupta: Progressive Delignification of Jute Fibre With ChlorineDioxide. J. Tect. Inst. (Trans), September 1951, pgs. T375-384,especially first page.

1. A SINGLE STAGE BLEACHING OPERATION FOR SULFATE CELLULOSE PULPS INWHICH THE ONLY BLEACHING OPERATION NECESSARY TO AFFORD PRODUCTS OFACCEPTABLE COLOR CONSISTS ESSENTIALLY OF SIMULTANEOUSLY CONTACTING THEPULP IN AQUEOUS MEDIUM UNDER BLEACHING CONDITIONS WITH CHLORINE ANDCHLORINE DIOXIDE, THE MOLAR RATIO OF CHLORINE TO CHLORINE DIOXIDE BEINGWITHIN THE RANGE OF ABOUT 2.5 TO ABOUT 3.5:1.